The structure of such aircraft needs to be compatible with standardized functional constraints, and also with certification, in particular.
In the event of the aircraft impacting against the ground (crashing), structural damage must be limited and even prevented, in particular in the vicinity of the cabin and the vital components of the aircraft. All unacceptable injury to humans must be avoided.
At present, complying with certain specifications is in contradiction with obtaining functions that are desired for such and such a component of the aircraft, thus requiring compromises to be made that are not always acceptable in practice. This applies in particular to rotary wing aircraft.
The search for increasing the capacity of the structure of an aircraft to absorb energy due to an impact (accident) has lead to numerous proposals.
Patent FR-2 632 604 describes a helicopter structure comprising a frame essentially constituted by a gantry whose ends are interconnected together by a cross-member. The cross-member is constituted mainly by a panel presenting a sandwich structure and constituted by two thin vertical cores defining a space that is filled with a deformable material (honeycomb or foam) to which they adhere. The cores of the panel present horizontal corrugations in their bottom portion. A particular structure makes it possible to obtain controlled deformation of the cross-member by initiating buckling in the event of an impact.
U.S. Pat. Nos. 4,084,029, 4,734,146, and FR-2 817 608 relate to beams having a composite corrugated core of sinusoidal shape. In FR-2 817 608, the beam is designed to absorb, in controlled manner, an intense and sudden compression force applied in its vertical direction under the effect of the kinetic energy released by a violent impact such as an aircraft crash. The core comprises a plurality of sheets of carbon fibers and of aramid fabric that are stacked together. Cuts formed in an edge adjacent to the soleplate of each fiber sheet serve to initiate rupture, so as to load and degrade the sheets progressively during application of a compression force suitable for causing the beam to rupture.
The present invention applies in particular to rotorcraft of a structure made up of frames of the kind described in FR-2 632 604, and also to beams interconnecting the frames in pairs.
Various other structures for absorbing energy in the event of an aircraft fuselage impacting the ground have also been proposed.
U.S. Pat. No. 6,620,484 describes an absorption structure comprising vertical composite panels having a thread stitched therein at a density that increases from the bottom of the structure towards its top portion.
U.S. Pat. No. 4,593,870 relates to an impact absorber that extends under the floor of a helicopter cabin and that comprises a crossed array of beams including a laminate or two laminates covering a honeycomb-structure core. Rupture-initiators are provided in the bottom of the laminate.
U.S. Pat. No. 4,941,767 describes intersecting planes with connections via ring-section parts. Patent FR-2 763 313 describes an installation for suspending a tank.
U.S. Pat. No. 5,069,318 describes reinforcement surrounding a thin wall in order to stabilize its behavior in the event of a crash. U.S. Pat. No. 5,451,015 describes an installation for protecting a tank in the event of a crash. U.S. Pat. No. 6,718,713 describes a preshaped member having an insertion channel for a plane partition. Document WO 03/018295 describes a member that is preformed to the shape of the Greek letter “Pi”, for assembling composite parts together.
In spite of that research, there remains a need for an improved aircraft structure for taking up forces and absorbing the energy that results from an impact.
In particular, it is appropriate when making such a structure safe in the event of a crash for the necessary adaptations to avoid major modification to the surroundings of the force take-up structure (whether pre-existing or being designed). The additional on-board weight and bulk should be small or negligible and the dynamic effects generated (buckling, movement of on-board equipment) should not be penalizing during normal operation of the aircraft. It is also desirable for installation and maintenance costs to be low, and for the means implemented to be simple, robust,and long-lasting.
These criteria are particularly draconian for rotary wing aircraft where questions of on-board weight and bulk are particularly constraining.
In particular, it is desirable to obtain an aircraft structure that is compatible with an impact speed of about 7 meters per second (m/s) to 9 m/s, e.g. about 8.2 m/s. It is also desirable for the maximum impact forces on the aircraft structure (e.g. the cabin floor) at the end of a crash to be compatible with the strength of the surrounding structure so as to avoid damaging it.
Up to some acceptable value, e.g. up to a value of the order of 25,000 newtons (N), the impact forces of a crash should not be transmitted to the take-up structure and their energy should be absorbed.